Heinlein complained that it would be like "revising Romeo and Juliet to let the young lovers live happily ever after." He also declared that changing the end "isn't real life, because in real life, not everything ends happily."Two things. First, Podkayne was killed because, after escaping from the terrorist hideout, she suddenly remembers she left a little Venusian fairy-baby behind. She goes back to get it...boom. This in itself is a plot gimmick to get her killed. No bright teenage girl of my acquaintance would take more than two steps before remembering to bring the fairy-baby along. Seriously, think about it. Just about any person who's owned a pet would, BEFORE escaping a place, make sure their pet was with them. We're talking pets here, too, not a helpless semi-intelligent infant. In the real world, Podkayne and her uncle would have brought the fairy-baby along. Her brother Clark might not have, but as he's portrayed, he's a borderline psychopath. You have to let your characters act as they want to, not shoehorn them in with clunky plot gimmicks. This is dedicated clunky, too, not ordinary lack of care. In just about how many thousands of movies, novels, TV shows, and comics does this little piece of trash appear? Hero forgets someone/something, has to go back, gets into trouble. Bah.
I might add that I remember reading this book as a child. The re-written ending had quite an effect on me. Podkayne was seriously injured in the explosion, but she and the fairy-baby lived. Seeing her hurt affected a change in Clark's personality. He dedicated himself to raising the fairy-baby, because it will please his sister when she recovers.
Here lies a secret I touch on in Suzie's Technical Support. I'll let Suzie explain: "Nothing's ever like a movie. People get stabbed, they get shot, they get blown up, but they aren't dead like they are in the movies. They're torn up, they hurt, they bleed, they moan, they scream." I knew this from my own childhood, from my own injuries, from my long days in a hospital. The revised ending wasn't a cop-out. It told a truth to those who read it. Life can fuck you up so much you'll wish you were dead...but you won't be.
Second. "It isn't real life because in real life, not everything ends happily." Happy endings are a cliché...but so are "real life" endings. What isn't a cliché, letting your characters tell you how the story ends.
Lastly, it isn't professional to tell your publisher you are writing a juvenile SF adventure novel...and give them noir instead. This is another example of the petty egotism some writers exhibit once they begin to have a little success. If ever I become successful, remind me of this essay.